For many years it has been well known to employ natural wood pulp fibers in the manufacture of the absorbent pad or core of disposable products such as diapers, sanitary napkins, surgical dressings, and the like. In the most general sense, there are but two basic processes, chemical and mechanical, for producing pulp fibers from natural wood. The characteristics of the pulp produced by the two basic processes differ considerably and, depending upon the intended final use to be made thereof, each has certain advantages and disadvantages.
In chemical wood pulping, there is a total or partial digestion and removal of the hydrophobic constituents of the wood, such as, lignin, carbohydrates and other nonligneous materials. The yield of chemical plup is predictably low and expensive, on the order of around 50%.
Mechanical pulping processes are more cost efficient, producing yields on the order of 90% and higher. Understandably, mechanical wood pulp, sometimes known as refiner pulp, is substantially hydrophobic due to the presence of lignin and other non-absorbing materials.
More recently, there has been increasing use of wood pulp produced by thermo-mechanical processes. Thermo-mechanical pulp (TMP) is essentially mechanical pulp, but has modified qualities because of an additional heating step. The thermomechanical process involves a step of first heating the wood chips to about 270.degree. F., usually with steam, to soften them for further mechanical processing. This heating stage tends to soften but not remove the lignin and also to loosen the individual wood fibers to ease actual defibration. Thermo-mechanical pulp thus has somewhat longer fibers than plain refiner pulp and produces structures of higher loft and greater flexibility.
Non-delignified wood pulp fibers, such as the thermo-mechanically produced wood pulp fibers, refiner produced wood pulp fibers, or the like, have become quite important in the last few years. These wood pulp fibers, also referred to as "high yield" wood pulp fibers, have become increasingly important for several reasons. The processes used to produce the fibers not only utilize more of the raw material than typical chemical processes, but the non-delignified wood pulp processes also reduce the environmental problems caused by chemical processing. Specifically, the "high yield" processes cause considerably less air pollution and water pollution than do the counterpart chemical processes. These various factors and the concomitant economic considerations make the high yield processes, such as the thermomechanical pulp process, very attractive.
Non-delignified wood pulp processes have been known for some time and are usually developed primarily for paper grade wood pulps, newsprint, and the like. These wood pulps have not been well accepted in absorbent type products, such as sanitary napkins, disposable diapers, and the like, primarily because of their relatively poor performance as the absorbent core for such products.
Conventional chemically processed wood pulp fibers have a degree of cohesive strength when placed in an air-laid web structure. Typically chemically processed wood pulp fibers are somewhat collapsed and appear in ribbon-like form. This form permits fiber entanglement during the air-laid web processing and hence results in a web having a degree of cohesiveness and fibrous web integrity.
In contrast, the non-delignified wood pulp fibers are non-collapsed, stiffer and more resilient. Webs formed of these fibers, although possessing a greater potential liquid holding capacity, have poor integrity and hence tend to break apart.
Furthermore, absorbent structures made from non-delignified wood pulp fibers are substantially hydrophobic and not readily wettable. For any absorbent structure to be satisfactory, it is highly desirable for the structure to (1) readily accept liquid, (2) easily transport the liquid from one portion of the structure to another, and (3) hold the liquid accepted.
Various techniques have been developed or suggested for improving the absorbent characteristics of non-delignified wood pulp, such as removing the fines from the wood pulp product or providing various solvent or other chemical treatments to the wood pulp product to both bleach the pulp and improve its absorbency. However, these techniques increase the economics or cost of the wood pulp and, in some instances, increase the pollution problem and, hence, do not take full advantage of the non-delignified wood pulp process.
Development of the use of mechanical wood pulp and thermo-mechanical pulp and some of the problems encountered in such use may best be appreciated by reference to some illustrative prior art examples. In "Mechanical Pulp In Absorbent Qualities", published by The Norwegian Pulp and Paper Institute (Sept., 1973) E. Bohmer et al describe the possible use of plain refiner or thermo-mechanical pulp in place of chemical pulp on a basis of cost, but conclude that it cannot achieve the liquid-holding capacity of chemical pulp. In "Thermo-Mechanical Pulp For Diapers, Other Absorbent Products" (Nov. 1975) Weyerhaeuser Company describes its new thermo-mechanical process for making pulp called Eco-Fluff and some of its potential uses. Among U.S. Patents: U.S. Pat. No. 4,047,531 teaches a two-layer pad, one of mechanical or thermo-mechanical pulp and the second of thermo-mechanical or chemical pulp; U.S. Pat. No. 4,120,747 teaches an absorbent paper made of thermo-mechanical or chemi-thermo-mechanical pulp; and U.S. Pat. No. 4,215,692 teaches an absorbent structure comprising a mixture of mechanical wood pulp (thermo-mechanical or refiner) and peat.
Other techniques for developing absorbent products utilizing non-delignified wood pulps have been suggested. One technique is disclosed in British Pat. No. 1,500,053 and uses fibers of specific measurement; that is, length and diameter. The surface-hydrophilicity of the fibers is increased by bleaching and the hydrophilic fibers are air-laid in web form and compressed to a specific density. Bleaching followed by compression substantially increases the wettability of the otherwise hydrophobic structure, but at the same time, reduces the liquid holding capacity of an absorbent structure made from non-delignified wood pulp fibers.
As mentioned above, for any absorbent structure to be satisfactory, it is not only necessary for the structure to hold liquid but also to readily accept liquid and transport it. The liquid holding capacity of the absorbent structure relates to the pore size of the fibrous bed and the wet bending modulus of the fibers. If the pore size (i.e., the spaces surrounding the fibers) is large and the wet bending modulus (i.e., stiffness) of the fibers is high, then the structure will have a relatively high liquid holding capacity but generally does not transport (wick) liquid readily. On the other hand, if the pore size is smaller and the bending modulus relatively low, the structure readily wicks liquid but will have a lower liquid holding capacity.
The fibers from the non-delignified wood pulp process can provide an absorbent structure having a large pore size and a high wet bending modulus of the fibers, however, such absorbent structures do not readily accept liquid, nor will the structure be readily densified or embossed to promote wicking.
As indicated by the cited illustrative references, the numerous efforts in this highly developed art to provide an absorptive structure utilizing cost efficient and desirable thermo-mechanical pulp are beset with difficulties that remain unsolved. These difficulties are partially or completely overcome by the present invention.